In recent years, a technique for forming transistors using a semiconductor thin film (with a thickness of several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. The transistors are widely used for electronic devices such as ICs or electro-optical devices.
Further, a technique in which a transistor including an oxide semiconductor is manufactured and applied to an electronic device or an optical device has attracted attention. For example, a technique of manufacturing a transistor by using zinc oxide or In—Ga—Zn—O-based oxide as an oxide semiconductor, and using the transistor for a switching element of a pixel of a display device and the like is disclosed in Patent Document 1 and Patent Document 2.